The invention relates generally to the field of motor controllers. In particular, this invention relates to an active speed regulating and current-limiting motor controller for dc motors and methods thereof.
A direct-current (xe2x80x9cdcxe2x80x9d) motor operates at a rotational speed that varies with changes in a load applied to the dc motor and an input voltage applied to an input for the dc motor. Electronic control units (xe2x80x9cECUxe2x80x9d) coupled with the input to de motor are commonly used to regulate the speed of the dc motor. The ECU controls the speed of the dc motor by controlling the input voltage in response to rotational speed of the dc motor. The ECU monitors the speed and adjusts the input voltage to compensate for differences in the speed from a programmed desired speed.
In a conventional configuration, the ECU is coupled with electronic sensors that generate signals associated with the actual speed of the dc motor. The ECU compares the actual speed with a desired speed programmed into the ECU. When the ECU determines that the actual motor speed differs from the desired motor speed, the ECU calculates a new input voltage that will compensate for the difference according to known motor control algorithms. The ECU then varies the input voltage to the calculated input voltage to adjust the motor speed towards the programmed motor speed.
Input current to the dc motor also depends on the load on and the speed of the dc motor. As a load on the dc motor increases, the input current to the dc motor also increases. When motor speed becomes too large relative to an existing load on the dc motor, the input current to the dc motor also increases. To prevent damage to the dc motor and input circuitry for the dc motor, the input current man be limited to a maximum.
One method for limiting current to the dc motor includes implementing hardware devices that disconnect the input voltage and input current to the dc motor when it is determined that the input current exceeds a maximum value. Hardware implementations may include blowing a series fuse or resettable circuit breaker when the input current exceeds the maximum. Other hardware implementations include opening an in-line transistor when the maximum current is detected. Because the dc motor is completely shutdown, the hardware implementations may be undesirable for certain applications. For example, restart of the dc motor may require a lengthy restart procedure, requiring an undesirably long down time. In other applications, continuous operation of the dc motor is critical, so the maximum current level is set at a conservatively high level and may not afford adequate protection for the dc motor and input circuitry.
An ECU may be programmed to limit the input current using a dc control feedback. The technique includes measuring the input current to the ECU while varying the input voltage to control the dc motor speed. When a maximum input current is exceeded, the ECU varies the input voltage to decrease the input current. Because the input voltage is decreased, the speed of the dc motor also decreases, which the ECU is programmed to control. The ECU will then detect the decreased speed and compensate by increasing the input voltage. Accordingly the ECU may cycle the input voltage up and down until the speed equalizes back to the desired speed. Accordingly, an ECU programmed for dc control feedback contradicts the objectives for speed regulation.
Another method for dc current control includes programming the ECU to digitally sample the input current and adjust the input current based on the digital sampling. However, because the input current varies faster than the dc motor speed, the sampling frequency for speed regulation may be too low to also effectively control the input current dynamics. Increases in the sampling rate to provide effective control for the input current dynamics increase the costs in providing dc motor control. In addition, programming the ECU with a dual sampling rate increases the complexity of the dc control and also increases the cost for the dc control.
Accordingly, there is a need for an ECU to provide active current-limiting control for dc motors while providing speed regulation for the dc motor with a relatively low and efficient sampling rate.
An embodiment of the motor controller includes a programmable electronic control unit (ECU) configured to control the rotational speed of the motor under varying load conditions and additionally to actively limit an input current to the dc motor. The ECU is configured to provide speed control for the dc motor while actively providing overcurrent protection for the dc motor. The motor controller provides a number of advantages over prior motor controllers that provide overcurrent protection through the use of hardware devices, such as relays or thermo cutoffs, and software techniques, such as direct current control algorithms.
In one embodiment for the active speed-regulating and current-limiting dc motor controller includes a speed sensor, a current sensor coupled with an input to the dc motor, the current sensor, and an electronic control unit (xe2x80x9cECUxe2x80x9d). The speed sensor may be mechanically coupled with the dc motor and configured to generate an electrical signal associated with a rotational speed of a dc motor. The speed sensor includes a speed sensor output, at which the electrical signal is provided. The current sensor may also be coupled with a dc motor input. The current sensor is configured to generate an electrical signal associated with a current level at the dc motor input. The current signal is provided at a current sensor output.
The ECU includes a first input, a second input, and an output. The first input is coupled with the speed sensor output and configured to receive the speed signal and the second input is coupled with the current sensor output and configured to receive the current signal. The ECU is operative generate a motor control signal at the output. The motor control signal is based on a comparison of the speed signal at the first input with a programmed desired rotational speed. The ECU is programmed to modify the control signal based on a comparison of the current signal at the second input with a programmed input current limit range. When the ECU determines that the input current is within or exceeds the input current limit range, the ECU modifies the control signal by a control decay factor. The ECU may include any programmable proportional integral derivative (PID) control unit device capable of accepting input signals from sensors, comparing the input signals with predetermined values, and generating an output signal based on the comparison of the input signals to the predetermined values. The control decay factors may be selected to provide the desired current limitations for the specific application.
An embodiment for a method for regulating dc motor speed includes the acts of: monitoring a rotational speed of the dc motor to determine whether the speed complies with a desired speed; generating a motor control signal in response to the determination comparison of the speed signal and a desired speed; monitoring an input current to the dc motor to determine whether an input current to the dc motor complies with an input current limit range; and modifying the motor control signal by a first control decay factor when it is determined that the input current exceeds the input current limit range. The method may further include modifying the motor control signal by a second control decay factor when it is determined that the input current is substantially within the input current limit range.
The foregoing discussion of the summary of the invention is provided only by way of introduction. Nothing in this section should be taken as a limitation on the claims, which define the scope of the invention. Additional objects and advantages of the present invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.